Phase transfer catalysts are used in a wide variety of chemical processes where one or more phase boundaries exist and one or more constituents cross a phase boundary. A phase transfer catalyst is capable of taking one reactant from one phase and transferring it into another phase in which a second reactant is located and in which the first reactant, after transfer, is in a reactive form such that a reaction between the two reactants can occur. Following the reaction of the two reactants, the phase transfer catalyst is then recycled to the first phase to transfer the first reactant to the second phase in order to catalyze the reaction of another reactant molecule. Such phase transfer catalysts are described in Starks, C., Liotta, C., Halpern, M.; “Phase Transfer Catalysis” Fundamentals, Applications and Industrial Perspectives”, Chapman and Hall, 1994, incorporated herein by reference.
Thus, phase transfer catalysis facilitates intimate contact of reactants that would not normally interact efficiently, usually because of phase solubility limitations. Phase transfer catalysis allows these reactions to proceed quickly, at low temperatures, and sometimes with great selectivity. A generalized example of how most PTC systems work is shown in FIG. 1 and is also known as the Starks Extraction Mechanism. In this system, the phase transfer catalyst, which may be a quaternary ammonium salt, is denoted as Q+Y−. The chemical species to be recovered is a water-soluble nucleophile, such as an anion denoted as X−. The organic substrate is identified as R-Y and typically is soluble in an organic solvent and not soluble in water hence the need for a specific catalyst system to bring the reagents together. The final product is denoted as R-X. In this organic-aqueous liquid—liquid system, the positively charged catalyst cation (Q+) pairs in the aqueous phase or aqueous-organic interface with the water-soluble anion or anion located at a solid surface (X−), and this complex (Q+X−) reaches equilibrium across the aqueous-organic interface or solid-organic interface, as shown in FIG. 1. In this way, the positively charged catalyst cation (Q+) delivers the anion to the organic phase, where it undergoes an irreversible reaction with an organic substrate (R-Y) to produce the desired product R-X.
More specifically, once formed, the ion pair (Q+X−) is a large organic species and distributes between the aqueous and organic phases. Once the phase boundary has been crossed, the reacting anion (X−) reacts in an essentially irreversible reaction with a neutrally charged organic substrate affecting the desired conversion and liberating the leaving group anion, Y−, which pairs with the quaternary ammonium cation to form the ion pair Q+Y− as mentioned above and shown in FIG. 1. It should be noted that, in some phase transfer catalysis systems, the complex (Q+X−) does not move into the organic phase but undergoes the reaction with the organic substrate at the organic-aqueous interface. The ion pair, Q+Y− comprising the quaternary ammonium catalyst and the leaving group anion, then migrates back to the aqueous phase by a reversible mechanism (either from the organic phase or the aqueous-organic interface) and can equilibrate with another target anion to regenerate Q+X−.
Many neutral and anionic nucleophiles can participate in phase transfer catalysis reactions and are also of interest to the subject invention. A long but not exhaustive list of anions includes these compounds and their derivatives: cyanide (CN−), thiocyanate (SCN−), cyanate (OCN−), hydrogen sulfide (HS−), sulfide (S2−), carbonate (CO32−), hydrogen carbonate (HCO3−), all thiocarbonates (monothio, dithio, and trithio), azide (N3−), sulfite, hydrogen sulfite, sulfate, hydrogen sulfate, thiolate (RS−), nitrite, nitrate,, hydrogen selenide (HSe−), selenide (Se2−), benzenesulfonate, chloride, bromide, fluoride, iodide, trichloroacetate (CCl3COO−), thiosulfate, thiophosphorate, chlorate, hypochlorite, malonate, dichloroacetate, chloroacetate, terephtalate, adipate, lactate, silicates, bromate, periodate, performate, m-chloroperbenzoate, formate, acetate, propionate, butyrate, benzoate, furoate, oxalate, phthalate, hydrogen phtalate, phenolate, cresolate, catecholate and many more. In this context, the term “derivative” means a compound which contains one of the nucleophilic groups listed above.
Existing treatment processes of cyanide waste streams include oxidation by bleaching, electrolytic oxidation, ozonation, air oxidation, and ion exchange. Most of these processes are based on oxidation. Recently, a novel oxidation method has been described by A. Alicilar, et al. The authors indicate that 86% yield of cyanide anions removal can be achieved at 60° C. It would be desirable to develop a method which offers significantly higher yields of cyanide removal.
As an example, one system which has utilized phase transfer catalysts is the formation of alkyl or acyl cyanides (nitriles), RCN or RCOCN, from sodium cyanide, NaCN, and alkylating or acylating agents, RY, RY2 and RCOY. Traditionally, aryl or alkyl cyanides have been prepared by reacting purchased or captively manufactured cyanide, CN−, HCN in the form of its ionized metal salt, CN− M+, with purchased alkylating agents, RY, such as epichlorohydrin, allyl chloride, benzyl chloride, benzyl bromide, methyl chloride, and ethyl chloride, methyl bromide, ethyl bromide, methyl iodide, ethyl iodide, other alkyl halides and alkyl sulfonates such as dimethyl and diethyl sulfate, alkyl carbonates such as dimethyl or dipropyl carbonate and alkyl arenesulfates such as alkyl benzenesulfonates and tosylates, etc. Acyl cyanides can be prepared from the corresponding acyl halides (such as benzoyl chloride) and sulfonyl halides (such as p-toluenesulfonyl chloride). The technology is also usable for dihalides, RY2, such as 1,4-dichlorobutane or 1,4-dichloro-2-butene.
Due to the inherent inefficiencies in many chemical processes, either HCN or NaCN, RY or RY2 is either formed or allowed to become a byproduct in the event of incomplete conversion. As used herein, a non-product stream shall mean a stream which contains at least one byproduct or waste and is separate from the final product stream of a process. HCN is a volatile chemical and is either directly incinerated producing greenhouse gases and nitrous oxide pollution or scrubbed out of gas streams with caustic thus producing water soluble NaCN. Recovery of NaCN or RY may be performed by using elaborate extraction systems. Such recovery of NaCN suffers from several disadvantages, including the requirement to use an excess of inert solvent for extraction or ion exchange resin for adsorption. The expense required for extraction and recovery of CN− increases even more when the aqueous streams containing CN− are less concentrated. The expense of recovering RY may come from it being hard to distill from a compound with a close boiling point or in the case of methyl chloride or methyl bromide, the need to cryogenically compress and store these volatile gases as liquids. Similar arguments can be made for the recovery of carboxylate or phenolate anions which would take the place of cyanide in the preceding discussion.
In order to convert the recovered, purchased or manufactured HCN into RCN, it must be transformed to its salt form, M+CN−, in which M+ is typically an alkali metal such as Na+, K+, or Li+. Also, in order to convert the recovered, purchased or manufactured RY or RY2 into RCN or R(CN)2, it must be brought into intimate contact with the anion CN− for reaction. The conversion of HCN is typically performed by contacting concentrated aqueous solutions of metal hydroxides (e.g., NaOH, KOH, LiOH) with HCN to yield M+CN− in concentrated solution, in solid form, or as a slurry. For example, cyano compounds=nitrites are prepared by reacting purchased HCN with benzyl chloride in the presence of aqueous sodium hydroxide and a catalyst.
Other purchased, recovered or manufactured compounds capable of forming anions which are commercially reacted with alkyl halides include carboxylic acids, cyanides, phenols, p-t-butylphenol, bisphenol A and resorcinol. Alkylating agents commercially reacted with RCOOH, NaCN, ArOH include ethyl chloroacetate and alkyl chlorides, such as methyl chloride (often formed from methanol and hydrogen chloride), ethyl chloride, and allyl chloride and alkyl bromides and iodides.
It would be desirable to develop a system which overcomes the disadvantages of purchasing, manufacturing, or recovering chemical species, such as hydrogen cyanide, HCN, carboxylic acids and their salts such as chloroacetic acid or phenol and its derivatives, ArOH, or RY or RY2. In addition, it would also be desirable to develop a system which minimizes the use of an organic solvent or eliminates the need for an organic solvent altogether. Moreover, it would be desirable to utilize a system which also extracts chemical species, such as HCN from a dilute non-product stream which would have the advantages of producing a product with some value from a non-product and avoiding the need to treat the stream for that non-product. In an analogous fashion, it would also be desirable to utilize a system which could react chemical species, such as methyl chloride or allyl bromide, from a gas or liquid non-product stream which would have the advantages of producing a product with some value from a non-product and avoiding the need to treat the stream for that non-product. A process which operates continuously also would be desirable in that its automated operation would decrease the costs associated with labor and special handling of batch operations.